In these years, liquid crystal display devices, which have rapidly become popular in replacement of cathode-ray tubes (CRT), are widely used in such as TVs, monitors, and mobile phones by making use of characteristics such as energy saving, thin body, and lightweight. Further, in some cases, an illumination device (i.e., a backlight) is provided in the rear of the liquid crystal display devices in the aim of spreading the use of the liquid crystal display device and improving display quality of the liquid crystal display device.
In order to achieve the improvement in the display quality and the like, upgrading of the illuminating device is desired.
The illuminating device can be classified mainly in a side-light type (edge-light type) and a direct type.
(Side-Light Type)
A side-light type illuminating device is an illuminating device including: a light guide plate; and a light source, wherein: in a case where the illuminating device is included in a liquid crystal display device, the light guide is positioned in the rear of a liquid crystal display panel of the liquid crystal display device; and the light source is provided on a lateral end section of the light guide.
In the side-light type illuminating device, light entered from the light source spreads over the entire light guide while repeatedly being reflected in the light guide, thereby indirectly illuminating the entire liquid crystal display panel evenly.
The side-light type configuration allows, even though a luminance is not so high, the illumination device to become thin and to have excellent uniformity of in-plane luminance. Accordingly, the side-light type illuminating device is used in small-and-medium-sized liquid crystal display devices such as a mobile phone and a laptop.
(Patent Literature 1)
Patent Literature 1 discloses an example of the side-light type illuminating device. That is, Patent Literature 1 discloses a surface emitting device in which a plurality of dots are formed on a reflective surface of a light guide plate so that light can be evenly emitted from a light emitting surface. In the surface emitting device, a corner section of the reflective surface does not become bright because light is not transmitted thereto due to a directivity of the light source. For avoiding the section being not bright, a density of dots in the corner section is higher than the other sections.
(Direct Type)
On the other hand, the direct type illuminating device is, unlike the side-light type illuminating device, an illuminating device in which a plurality of light sources are arranged in the rear of a liquid crystal display panel so as to directly illuminate the liquid crystal display panel.
The direct type illuminating device makes it possible to easily obtain a high luminance in a large-sized screen, thereby being used mainly for a large-sized liquid crystal display device (liquid crystal display) which has a size of 20 inches or more.
However, the current direct type illuminating device has a thickness of approximately 20 mm to 40 mm, and the thickness prevents the liquid crystal display from becoming thinner.
Actually, it is possible to achieve a thinner large-sized liquid crystal display by shortening a distance between the light source and the liquid crystal display panel. In that case, a larger number of the light sources are required for obtaining a uniform luminance of the illuminating device, and such increase of the number of the light sources leads to increase of the cost. In view of the disadvantage, it is desired to develop a thin illuminating device having excellent uniformity of luminance, without increasing the number of the light sources.
Conventionally, in order to solve the problem, it has been tried to achieve a thinner large-sized liquid crystal display by arranging a plurality of side-light type illuminating devices.
(Patent Literature 2)
For example, Patent Literature 2 discloses a surface emitting device which can achieve light emission in a large area while having a compact structure, thereby being suitable for use in a large-sized liquid crystal display. The surface emitting device has a tandem structure in which plate-like light guide blocks are tandemly arranged, and primary light sources are included for supplying primary light to the respective light guide blocks.
The illuminating device, as described above, is called as a tandem type illuminating device in which a plurality of light emitting units, each of which includes a combination of a light source and a light guide, are arranged.